You can specify normals for faces you create. This is a vector that's perpendicular to the face. Specifying normals is important if you plan to use lighting. They tell OpenGL how the light is to be reflected. Although OpenGL is not a raytracer, it still influences how bright you see the face. OpenGL will not calculate the normals itself, you have to do it manually.
Specifying Normals[edit | edit source]
Normals are specified using the function glNormal3f and used before you make the vertices that belong to the face. Example:
// Create a square on the XY axis. The normal vector is then (0, 0, 1) glBegin(GL_QUADS); glNormal3f(0.0, 0.0, 1.0); glVertex3f(0.0, 0.0, 0.0); glVertex3f(0.0, 1.0, 0.0); glVertex3f(1.0, 1.0, 0.0); glVertex3f(1.0, 0.0, 0.0); glEnd();
Calculating Normals[edit | edit source]
There is a quite simple approach for calculating the normal vector for a triangle. Given p1, p2 and p3, the normal vector can be expressed as the cross product of (p2 - p1) and (p3 - p1), divided by the length of the result. In C code, using a struct type for the vectors:
struct vector a, b, n; double l; a.x = p2.x - p1.x; a.y = p2.y - p1.y; a.z = p2.z - p1.z; b.x = p3.x - p1.x; b.y = p3.y - p1.y; b.z = p3.z - p1.z; n.x = (a.y * b.z) - (a.z * b.y); n.y = (a.z * b.x) - (a.x * b.z); n.z = (a.x * b.y) - (a.y * b.x); // Normalize (divide by root of dot product) l = sqrt(n.x * n.x + n.y * n.y + n.z * n.z); n.x /= l; n.y /= l; n.z /= l; // Now pass n.x, n.y, n.z to glNormal3f